1. Technical Field
The present invention relates generally to a structure for and method of operating an off-chip driver. More particularly, the present invention relates to an improved transition-controlled off-chip driver dissipating zero DC power.
2. Background Art
In recent years, there has been an increase in the need for off-chip drivers (OCD) that consume little power, offer predictable performance under varying process and operating conditions, and eliminate on-chip noise caused by the driver. This is particularly true with respect to microprocessors in the lap-top computer market where battery power conservation is required, while not sacrificing performance qualities.
A recent innovation in driver circuits, in particular those using CMOS technology, sought to provide predictable performance of such circuits in the face of widely varying process and operating conditions. This was accomplished by controlling transitions of output devices within driver circuits by compensating for best and worst case performance, i.e., fastest and slowest response times, respectively.
Such a transition control technique is described in U.S. Pat. No. 4,975,599. There, additional circuitry is used to oppose a fast turn-on of driver output devices and enhance a slow turn-on. Output feedback is used to disable compensation after transition is complete.
While the above scheme effectively controls such off-chip driver performance, it is not optimum where zero DC power dissipation and noise elimination are desired, as in the lap-top computer market. First, DC power dissipation from supply voltage to ground and in several transistor paths exists. Second, the use of feedback to end control after transition is complete introduces noise into the chip.
Other prior art transition control techniques exist, such as described in U.S. Pat. No. 5,017,807, that control transition and noise by using a resistor-capacitor network. However, such techniques dissipate DC power and control turn-off as well as turn-on of driver output devices. Control of turn-off may cause both output devices to be on simultaneously, which wastes unnecessary power.
While the prior art does provide effective transition-control techniques for more uniform performance of off-chip drivers, these techniques dissipate unnecessary DC power, control both turn-on and turn-off of driver output devices, potentially causing them to be on simultaneously, and introduce unnecessary noise by using feedback. Microprocessors in the lap-top market, may not operate optimally under such conditions. Thus, a need exists for off-chip drivers that dissipate zero DC power, control turn-on of output devices, but not turn-off, and eliminate output feedback noise.